1. Field of the Invention
The present invention relates to a system for implanting ions on a target material, and more particularly but not by way of limitation, to a control system for adjusting an electron shower output to maintain substantially continuous control of disk current during ion implantation.
2. Description of the Prior Art
In the fabrication of semiconductors, an implanter is used to deposit ions into a material which determines the conduction properties of the silicon disk wafer. A typical implanter consists of an ion source, an analyzer, a Faraday flag, an electron shower, and a disk assembly. The ion source supplies the ions to be implanted. These ions can be of several different chemical elements and pass through a magnetic field in the analyzer. Based on the mass to charge ratio of the generated ions, the analyzer selects certain ions to reach the target wafer for implantation. The Faraday flag is a monitoring assembly for measurement and setup prior to implanting. Typically made of graphite, the Faraday flag is used to block the ion stream before implantation begins and is physically moved to allow the ions to reach the target wafer during implantation. In the closed position, the Faraday flag blocks the ion beam whose action causes emission of secondary electrons. To prevent loss of these secondary electrons which would result an incorrect measurement of beam current, magnets are attached to the Faraday flag to prevent the escape of secondary electrons. The electron shower neutralizes target water charging and the disk assembly holds the target wafers and scans them through the beam of ions.
If the surface of the target wafer is not conductive, the high energy beam of ions produced by the ion source causes a positive charge to accumulate on the surface of the target wafer. This charge may build up to sufficient levels to cause dielectric breakdown and thus damage devices on the target wafer. The electron shower minimizes the positive charging effect of the ion beam on the target wafer. Negatively charged electrons are generated by heating a filament in the electron shower which causes electrons to boil off and create secondary electrons in the vacuum. These electrons are attracted to the positively charged target wafer and neutralize the charge produced by the beam of ions.
Various control systems have been proposed to monitor the electrons generated and to control the filament current in order to maintain a preset voltage level or charge level on the target wafer. In one prior art control system the implanter is modified to electrically isolate the target wafer so that current passing through the target wafer to ground (the disk current) can be monitored. A positive disk current indicates that too few electrons are being generated and a negative disk current indicates that too many electrons are being generated. The output of the electron shower in such a control system is adjusted only once for the entire operation and, as a result of the uncontrolled charge accumulation, use of such control system does not completely eliminate the destruction caused by charge building up on the target wafer.
The above described prior art control system also suffers from another inherent defect which occurs when the electron shower is turned off once the desired implantation has occurred. That is, in operation the electron shower is turned off the instant implantation is to cease and, at that instant in time, the Faraday flag is closed to block the beam of ions. However, owing to the physical mass involved, the Faraday flag does not close instantly and the delay in closing permits positive charging and associated destruction of the target wafer.
The Faraday flag also causes another undesirable result. Because the Faraday flag has magnets on it, the motion of the flag causes the electrons from the electron shower to surge toward the target wafer. This surge produces an excessive negative disk current regardless of whether the Faraday flag is opening or closing. This surge produces negative charging and associated destruction of the target wafer.
Another prior art control system for controlling the charging of target wafer during and after implantation in an effort to prevent damage to the target wafer is a modification of the above-described control system. In the modified control system the disk current is adjusted at the start of each scan of the ion beam so that the electron shower is held constant for the entire scan. However, this requires complex and expensive apparatus and does not control the charging of the target wafer during the scan.
While these prior art control systems have met with some success, problems associated with undesired charging of the target water and the associated destruction of the target wafer at the end of implantation remain. Therefore, a need exists for a control system wherein the destructive effects of target wafer charging are substantially eliminated. It is to such a control system that the present invention is directed.